1. Field of the Invention
The invention is related to the field of radar systems, and in particular, to radar systems that transfer pulses at a target to determine the range and velocity of the target.
2. Statement of the Problem
A Doppler weather radar system determines the range and velocity of atmospheric structures, such as wind and precipitation. The Doppler weather radar system transfers pulse trains toward the target structure and receives echoes reflected off of the target structure. The Doppler weather radar system processes the echoes to determine the range and velocity of the atmospheric structure. In addition, the Doppler weather radar system determines other variables, such as differential reflectivity, differential phase, linear depolarization ratio, and co-polar correlation coefficient.
To determine the target range, the radar system determines the time that it takes for a pulse to propagate to the target and for its echo to propagate back to the radar system. The range is then determined based on the expected speed of the pulses/echoes and the elapsed time. However, the echoes from consecutive pulses may be confused by the radar system. For example, a first pulse may reflect off of the target to provide a first echo, and a second pulse may reflect off of the target to provide a second echo. However, the first pulse may continue to propagate and reflect off of another structure that is further away from the radar system than the target structure to produce a third echo. The third echo may arrive at the receiver around the same time as the second echo. The receiver may confuse the two echoes and provide an ambiguous or inaccurate target range. These confounded echoes are referred to as range overlaid echoes in the literature.
To counter the ambiguous range problem, it is desirable to separate consecutive pulses by a relatively long Pulse Repetition Time (PRT). The longer PRT allows all echoes from the first pulse to clear the radar system before the echoes from the second pulse arrive. This prevents the receiver from confusing the echoes of consecutive pulses.
To determine target velocity, the radar system determines the phase shift that occurs between two echoes returned from a common target, since the velocity of the target affects this phase shift according to the Doppler effect. To get proper phase shift data, the target must be sampled at the Nyquist rate or at a higher rate, and these rates require a shorter PRT to adequately sample for unambiguous target velocity.
The velocity determination can be further complicated by the introduction of phase shift that is not attributed to the velocity of the target structure. For example, additional phase shift is introduced to both the pulse and its echo by the particles that are in between the radar system and the target structure. Since this other phase shift is not the Doppler phase shift of interest, the other phase shift introduces ambiguity into the velocity determination.
Unfortunately, larger unambiguous range determination requires a longer PRT, but larger unambiguous velocity determination requires a shorter PRT. This problem of simultaneously obtaining larger unambiguous range and larger unambiguous velocity is termed in the literature as the Doppler Dilemma. It is expressed by the following equation:rava=cλ/8;where ra is the unambiguous range, va is the unambiguous velocity, c is the speed of light, and λ is the wavelength of the transmitted radar signal. Various techniques have been tried to simultaneously extend both the unambiguous range and velocity of the target, despite the conflicting pulse repetition requirements.
One technique to handle the range/velocity ambiguity problem is to use dual scans where the first scan has a short PRT for velocity determination, and the second scan has a longer PRT for range determination. Unfortunately, the dual scanning technique may take an undesirable amount of time to perform both scans, and range overlaid echoes can contaminate the short PRT scan.
Another technique to handle the range/velocity ambiguity problem is to alternate the period between the pulses between a short PRT and a long PRT, called a staggered PRT. The unambiguous velocity can be extended to an equivalent uniform PRT of the difference of the long and short PRT periods, while the unambiguous range can be extended to an equivalent uniform PRT of the sum of the long and short PRT. Unfortunately, complex filters are required to remove noise caused by ground clutter when the PRT is alternated.
Another technique to mitigate range-velocity ambiguity is to use systematic phase coding of the transmitted pulses. In this way, the various range overlaid echoes can be separated using the phase code information. This technique works well for two overlaid echoes, but becomes very difficult when three or more echoes are overlaid.
Another technique to handle the range/velocity ambiguity problem is to transfer a first pulse train at a first polarity and with a set pulse repetition time, and to transfer a second pulse train at a second polarity, but with an alternating PRT. The alternating PRT means that, for the second pulse train, a first pulse is transmitted slightly before a corresponding first pulse in the first pulse train, and a second pulse is transmitted slightly after a corresponding second pulse in the first pulse train. Thus, pulses in the second pulse train alternate between transmission slightly before or after pulses in the first pulse train. Unfortunately, the continually alternating PRT adds undesired complexity to ground clutter filtering and to phase coding.